Aluminum electrolytic cell method with application of refractory protective coatings on cello components

ABSTRACT

An adherent protective coating of a refractory material is produced on the surface of carbonaceous, refractory, ceramic, metallic or other materials serving as components of electrolytic cells operating at high temperature, by applying to such surfaces a well chosen micropyretic reaction layer from a slurry, which when dried is ignited to initiate a self-sustaining micropyretic reaction, along a combustion front, to produce condensed matter forming such refractory protective adherent coating. The slurry is preferably applied in several layers, the first layer(s) to facilitate adherence and the last layer(s) to provide protection and may contain some preformed non-reactant materials. The electrolytic cells whose components require such coatings are especially those operating at high temperature with a molten salt electrolyte, particularly those for the production of metals, aluminum being the most important. For these cells the invention provides an aluminum-wettable, adherent, refractory, protective coating for the cell-bottom carbon cathode, containing RHM material such as titanium diboride, as well as other refractory protective coatings for cell wall lining and for other cell components. Novel cell designs for the utilization of the different coatings are also provided.

This is a division of application Ser. No. 07/861,513 filed on Apr. 1,1992, now U.S. Pat. No. 5,310,476.

FIELD OF THE INVENTION

Generally, the invention relates to the development of a coatingtechnology to apply different compositions of refractory materials suchas those containing hard metals, particularly titanium borides, metallicalloys, intermetallic compounds, cermets, oxides, metals and ceramics tothe surface of substrates made of different materials such ascarbonaceous materials, refractory materials, ceramics, cermets, oxides,metallic alloys (particularly those of iron, nickel, aluminum, andcopper) and intermetallic compounds.

Such substrates may in particular be components of electrolytic cellsoperating at high temperatures, particularly aluminium production cells.The present invention thus more specifically relates to a novel methodof application of adherent protective coatings of refractory material tothe surface of substrates of components of electrolytic cells for moltensalt electrolysis for the electrowinning of metals and operating at hightemperatures, particularly for the production of aluminium and as wellto novel designs of such cells and their operation.

The protective coating is a refractory material or a combination ofrefractory materials containing aluminum-wettable hard metals,particularly titanium borides or other materials consisting of metallicalloys, intermetallic compounds, cermets, oxides and ceramics on thesurface of the substrates e.g. of electrolytic cell components, inparticular an adherent protective coating of aluminium-wettablerefractory material on the surface of a carbonaceous or refractorysubstrate lining the cell bottom floor of an aluminium production cell.

The invention also relates to composite materials comprising acarbonaceous or refractory substrate coated with an aluminium-wettablerefractory material and to the use of the coated composite materials insuch cells.

BACKGROUND OF THE INVENTION

Among the metals obtained in electrolytic cells operating at hightemperature in a molten salt electrolyte containing an oxide or compoundof the metal to be electrowon, aluminium is the most important and theinvention will describe in particular the protection of components ofaluminium cells, more particularly the protection of the cell cathodebottom by applying an aluminium wettable, adherent coating.

Aluminium is produced conventionally by the Hall-Heroult process, by theelectrolysis of alumina dissolved in molten salt containing cryolite attemperatures around 950° C. A Hall-Heroult reduction cell typically hasa steel shell provided with an insulating lining of refractory material,which in turn has a lining of carbon which contacts the moltenconstituents. Conductor bars connected to the negative pole of a directcurrent source are embedded in the carbon cathode substrate forming thecell bottom floor. The cathode substrate is usually an anthracite basedcarbon lining made of prebaked cathode blocks, joined with a rammingmixture of anthracite, coke, and coal tar.

In Hall-Heroult cells, a molten aluminium pool acts as the cathode. Thecarbon lining or cathode material has a useful life of three to eightyears, or even less under adverse conditions. The deterioration of thecathode bottom is due to erosion and penetration of electrolyte andliquid aluminium as well as intercalation of sodium, which causesswelling and deformation of the cathode carbon blocks and ramming mix.In addition, the penetration of sodium species and other ingredients ofcryolite or air leads to the formation of toxic compounds includingcyanides.

Difficulties in operation also arise from the accumulation ofundissolved alumina sludge on the surface of the carbon cathode beneaththe aluminium pool which forms insulating regions on the cell bottom.Penetration of cryolite and aluminium through the carbon body and thedeformation of the cathode carbon blocks also cause displacement of suchcathode blocks. Due to displacement of the cathode blocks, aluminiumreaches the steel cathode conductor bars causing corrosion thereofleading to deterioration of the electrical contact and an excessive ironcontent in the aluminium metal produced.

A major drawback of carbon as cathode material is that it is not wettedby aluminium. This necessitates maintaining a deep pool of aluminium (atleast 100-250 mm thick) in order to ensure a certain protection of thecarbon blocks and an effective contact over the cathode surface. Butelectromagnetic forces create waves in the molten aluminium and, toavoid short-circuiting with the anode, the anode-to-cathode distance(ACD) must be kept at a safe minimum value, usually 40 to 60 mm. Forconventional cells, there is a minimum ACD below which the currentefficiency drops drastically, due to short-circuiting between thealuminium pool and the anode. The electrical resistance of theelectrolyte in the inter-electrode gap causes a voltage drop from 1.8 to2.7 volts, which represents from 40 to 60 percent of the total voltagedrop, and is the largest single component of the voltage drop in a givencell.

To reduce the ACD and associated voltage drop, extensive research hasbeen carried out with Refractory Hard Metals (RHM) such as TiB₂ ascathode materials. TiB₂ and other RHM's are practically insoluble inaluminium, have a low electrical resistance, and are wetted byaluminium. This should allow aluminium to be electrolytically depositeddirectly on an RHM cathode surface, and should avoid the necessity for adeep aluminium pool. Because titanium diboride and similar RefractoryHard Metals are wettable by aluminium, resistant to the corrosiveenvironment of an aluminium production cell, and are good electricalconductors, numerous cell designs utilizing Refractory Hard Metal havebeen proposed, which would present many advantages, notably includingthe saving of energy by reducing the ACD.

The use of titanium diboride and other RHM current-conducting elementsin electrolytic aluminium production cells is described in U.S. Pat.Nos. 2,915,442, 3,028,324, 3,215,615, 3,314,876, 3,330,756, 3,156,639,3,274,093 and 3,400,061. Despite extensive efforts and the potentialadvantages of having surfaces of titanium diboride at the cell cathodebottom, such propositions have not been commercially adopted by thealuminium industry.

The non-acceptance of tiles and other methods of applying layers of TiB₂and other RHM materials on the surface of aluminium production cells isdue to their lack of stability in the operating conditions, in additionto their cost. The failure of these materials is associated withpenetration of the electrolyte when not perfectly wetted by aluminium,and attack by aluminium because of impurities in the RHM structure. InRHM pieces such as tiles, oxygen impurities tend to segregate alonggrain boundaries leading to rapid attack by aluminium metal and/or bycryolite. To combat disintegration, it has been proposed to use highlypure TiB₂ powder to make materials containing less than 50 ppm oxygen.Such fabrication further increases the cost of the already-expensivematerials. No cell utilizing TiB₂ tiles as cathode is known to haveoperated for long periods without loss of adhesion of the tiles, ortheir disintegration. Other reasons for failure of RHM tiles have beenthe lack of mechanical strength and resistance to thermal shock.

Various types of TiB₂ or RHM layers applied to carbon substrates havefailed due to poor adherence and to differences in thermal expansioncoefficients between the titanium diboride material and the carboncathode block.

U.S. Pat. No. 3,400,061 describes a cell without an aluminium pool butwith a drained cathode of Refractory Hard Metal which consists of amixture of Refractory Hard Metal, at least 5 percent carbon, and 10 to20% by weight of pitch binder, baked at 900° C. or more and rammed intoplace in the cell bottom. Such composite cathodes have found nocommercial use probably due to susceptibility to attack by theelectrolytic bath.

U.S. Pat. No. 4,093,524 discloses bonding tiles of titanium diboride andother Refractory Hard Metals to a conductive substrate such as graphite.But large differences in thermal expansion coefficients between the RHMtiles and the substrate cause problems.

U.S. Pat. No. 3,661,736 claims a composite drained cathode for analuminium production cell, comprising particles or pieces of arc-melted"RHM alloy" embedded in an electrically conductive matrix of carbon orgraphite and a particulate filler such as aluminium carbide, titaniumcarbide or titanium nitride. However, in operation, grain boundaries andthe carbon or graphite matrix are attacked by electrolyte and/oraluminium, leading to rapid destruction of the cathode.

U.S. Pat. No. 4,308,114 discloses a cathode surface of RHM in agraphitic matrix made by mixing the RHM with a pitch binder andgraphitizating at 2350° C. or above. Such cathodes are subject to earlyfailure due to rapid ablation, and possible intercalation by sodium anderosion of the graphite matrix.

To avoid the problems encountered with tiles and with the previouscoating methods, U.S. Pat. No. 4,466,996 proposed applying a coatingcomposition comprising a pre-formed particulate RHM, such as TiB₂, athermosetting binder, a carbonaceous filler and carbonaceous additivesto a carbonaceous cathode substrate, followed by curing andcarbonisation. But it is still not possible by this method to obtaincoatings of satisfactory adherence that could withstand the operatingconditions in an aluminium production cell. It has also provenimpossible to produce adherent coatings of RHM on refractory substratessuch as alumina.

U.S. Pat. No. 4,560,448 describes a structural component of an aluminiumproduction cell which is in contact with molten aluminium, made of anon-wettable material such as alumina which is rendered wettable by athin layer (up to 100 micrometer) of TiB₂. However, to preventdissolution of this TiB₂ layer, the molten aluminium had to bemaintained saturated with titanium and boron and this expedient was notacceptable.

U.S. Pat. No. 5,004,524 discloses a body of fused alumina or anotherrefractory oxycompound having a multiplicity of discrete inclusions ofTiB₂ or other aluminium-wettable RHM cast into its surface. Thismaterial is particularly suitable for non-current carrying cathodebottom floors of aluminium production cells, but in the long term evenif the material may remain bound to the fused alumina and resist tocorrosion, the manufacture at an acceptable cost remains a problem.

U.S. Pat. No. 4,595,545 discloses the production of titanium diboride ora mixture thereof with a carbide and/or a nitride of titanium,zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum ortungsten by carbothermic, carbo-aluminothermic or alumino-thermicreaction, under vacuum or an inert atmosphere, of a glass ormicrocristalline gel of oxide reactants prepared from organic alkoxideprecursors. This glass or gel was then ground and formed into bodies andsintered into bodies of titanium diboride/alumina-based materials ascomponents of aluminium production cells. But such sintered materialsare subject to attack and grain-boundary corrosion when in contact withmolten aluminium. Similar reactions, known as combustion synthesis,self-propagating high temperature synthesis or micropyretic reactionsare known (see below, under the heading "Micropyretic Reactions"), butto date these reactions have not been applied to the production ofrefractory coatings on carbonaceous, refractory or other substrates insuch a way, and with the right composition, as to lead to coatings withadequate adherence to survive the operating conditions in an aluminiumproduction cell.

U.S. Pat. No. 4,600,481 proposed making components of aluminiumproduction cells by infiltrating aluminium into a skeletalself-sustaining matrix of alumina or another refractory material whichis normally non-wettable by molten aluminium, after having rendered thesurface of the matrix wettable by molten aluminium for instance bytreating the surface with a wetting agent such as titanium diboride, inparticular with a titanium diboride composite material producedaccording to the previously-mentioned patent. In this case, only atemporary surface wetting was thought to be required to facilitate theinfiltration, but in practice it was not easy to produce materials thatsufficiently maintained the internal wetting to sustain long operatingperiods when the component was exposed externally to molten aluminium.Also, the described techniques have not been applied to externalsurfaces of refractory bodies to make them permanently wettable bymolten aluminium.

The methods employed to date have thus not successfully producedadherent protective coatings of refractory materials, in particularaluminium wettable refractory materials such as TiB₂ and otherRefractory Hard Metals, on various substrates and in particular oncarbonaceous or refractory substrates, that adhere to and remain firmlyattached to the substrate in conditions such as encountered in aluminiumproduction cells, the coating providing a permanent and perfectlyprotective surface that is wetted by molten aluminium.

SUMMARY OF THE INVENTION

The invention aims to overcome the deficiencies of past attempts toutilize refractory materials in particular Refractory Hard Metals assurface coatings on substrates, in particular but not exclusivelycarbonaceous, refractory and metallic substrates, for use generally forprotecting the substrates from the corrosive attacks of liquids andgases, inter alia for use as cell components for molten saltelectrolysis cells, especially for use as cathodes or other cellcomponents of aluminium production cells.

The invention relates in particular to the protection of the surfaces ofcomponents of electrolytic cells, particularly those operating at hightemperatures, from the attack of liquids and gases existing in the cellsor formed during electrolysis by applying a refractory coating byutilizing novel micropyretic methods. A refractory coating or refractorymaterial when mentioned in this description of the invention shall meana material, whether carbonaceous, ceramic, or metallic, which canwithstand high temperatures.

An object of the invention is to provide a method of producingrefractory materials, in particular aluminium wettable refractorymaterials, making use of a micropyretic reaction in a slurry-appliedreaction layer of such composition and so controlled that the method canproduce extremely adherent refractory coatings on carbonaceous,refractory, metallic or other substrates that can inter alia be used ascathodes in aluminium production or more generally as any cell componentwhere wettability with aluminium is desirable, as well as resistance tocryolite and oxidation. Other applications may make use of thematerial's excellent resistance to corrosion, in particular tooxidation, especially in high temperature environments.

The coating is obtained by applying to the surface of the substrate,e.g. of the component of the electrolytic cell which needs to be coatedand protected, a well chosen micropyretic slurry which when dried isignited to initiate a self-sustaining micropyretic reaction in the driedslurry, along a combustion front, to produce condensed matter forming acoating adherent to the surface of the substrate and protecting

The composition of the micropyretic slurry is chosen according to thephysical and chemical characteristics of the substrate and the purposeof the coating. The slurry is preferably applied in several layers, thefirst layer(s) to facilitate adherence and the last layer(s) to provideprotection.

The coatings obtained by the method according to the invention are welladherent to the different substrates, provide the required protection tothe cell components and have the desired mechanical, physical, chemical,and electrochemical characteristics.

The coatings are impervious and adherent to the substrates and resistantto thermal shocks therefore protecting the substrates efficiently fromthe corrosive attacks of liquids, fumes and gases existing or producedin electrolytic cells, thus making them ideal for use in molten saltelectrolysis cells, in particular those for aluminum production. In anelectrolytic cell operating at high temperature all cell components haveto be mechanically strong at the operating temperature and each one mayhave any additional required characteristic.

In the particular case of aluminium production cells, analuminium-wettable, refractory, electrically conductive, adherentcoating has been developed to be applied to the surface of the cellcathode bottom made of carbonaceous material to protect suchcarbonaceous material from the attack of sodium and air which producesdeformation of the cathode blocks and formation of dangerous nitrogencompounds such as cyanides.

By protecting the carbonaceous cell components from attack by NaF orother aggressive ingredients of the electrolyte, the cell efficiency isimproved. Because NaF in the electrolyte no longer reacts with thecarbon cell bottom and walls, the cell functions with a defined bathratio without a need to replenish the electrolyte with NaF.

The aluminum-wettable refractory coating will also permit theelimination of the thick aluminium pool required to partially protectthe carbon cathode, enabling the cell to operate with a drained cathode.Other coatings have been developed to protect the upper part of thecarbonaceous cell wall and cell cover and anode current feeders andholders from the attack of fluoride fumes and oxidation by oxygen or airand the lower part from the attack by the cryolite-containingelectrolyte.

Special coatings have also been developed to protect anode substratesfrom the attack of oxygen and cryolite.

The protective effect of the coatings according to the invention is suchas to enable the use of relatively inexpensive materials for thesubstrates. For instance, cheaper grades of graphite can be used insteadof the more expensive anthracite forms of carbon, while providingimproved resistance agains the corrosive conditions in the cellenvironment.

The composite materials resulting from coating substrates according tothe present invention can be utilized also as components of electrolyticcells for the production by molten salt electrolysis of other metalssuch as magnesium, sodium, potassium, titanium, and others, and also forcells operating at low temperatures and for the surfaces of any otherparts of electrochemical equipment requiring electrochemical, chemical,or physical stability.

The present invention concerns a method which is not only superior andless costly than other suggested, well-known methods such as plasma orflame spray, electrodeposition and dip coating, but in many cases is theonly applicable and efficient method.

According to the invention, a method has been developed for producing acomponent of an aluminium production cell which in operation of the cellis exposed to a molten electrolyte and/or to molten aluminium, whichcomponent comprises a substrate of carbonaceous or refractory materialor a cermet, a metal, a refractory oxide, a metallic alloy or anintermetallic compound coated with a coating of refractory material.This method comprises applying to the substrate a micropyretic reactionlayer from a slurry containing particulate reactants preferably in acolloidal carrier, and initiating a micropyretic reaction. Morespecifically, the invention relates to a method of producing arefractory adherent material by applying one or more layers of one ormore micropyretic slurries one or more of which contains particulatereactants, to a substrate and drying each of them before applying thefollowing layer, to provide on the substrate at least one dried layercontaining the particulate reactants. The slurry-applied layer is thenignited to initiate a self-sustaining micropyretic reaction in the driedlayer, along a combustion front, to produce condensed matter forming acoating of refractory material adherent to the surface of the substrateand protecting it.

To assist rapid wetting of the components by molten aluminium, therefractory material coated on the substrate may be exposed to moltenaluminium in the presence of a flux assisting penetration of aluminiuminto the refractory material, the flux for example comprising afluoride, a chloride or a borate, of at least one of lithium and sodium,or mixtures thereof. Such treatment favors aluminization of therefractory coating by the penetration therein of aluminium.Aluminization may also be assisted by including powdered aluminium inthe slurry of micropyretic reactants with optional non-reactive fillers.

The substrate of the component may be coated outside the aluminiumproduction cell and the coated component then inserted into the cell.Alternatively, the component is part of a cell which is coated in thecell prior to operation. For instance, the component is part of a cellbottom formed by an exposed area of carbonaceous material, an exposedarea of refractory material, an exposed area of a metal alloy, or anexpanse comprising exposed areas of carbonaceous material, refractorymaterial and/or metal alloys. In this case, the slurry is preferablyapplied to the cell bottom in several layers with drying of eachsuccessive layer, and the micropyretic reaction is initiated by a mobileheat source. The micropyretic slurry preferably contains the particulatereactants in a colloidal carrier, e.g. comprising colloidal silica,colloidal yttria, and/or colloidal monoaluminium phosphate in varioussolvents. This colloidal carrier may be in an aqueous solvent butadvantageously comprises an organic solvent, particularly anurethane-based solvent.

Particulate or fibrous non-reactant filler materials can be included byapplying one or more layers from a slurry of particulate non-reactantfiller materials or by including particulate or fibrous non-reactants inthe micropyretic slurry.

The substrate may be carbonaceous in which case it may be made ofanthracite based carbon or of graphite and other grades of carbon usedin aluminium production cells. Advantageously, use may be made fo thecheaper grades of carbon. Ceramic substrates include but are not limitedto alumina and other materials that are not normally wettable by moltenaluminium, such as aluminium nitride, aluminium oxynitride, boronnitride, silicon carbide, silicon nitride and aluminium boride. Otherceramics, cermets, metals such as copper and metallic alloys such assteel and cast iron or those of nickel, aluminium and copper can alsoserve successfully as substrates utilizing the present invention. Thesubstrates may be bodies or tightly packed agglomerates. The substratesmay have a microporous surface providing anchorage for the appliedaluminium-wettable refractory material. Thus, sintered or tightly packedsubstrates may sometimes be preferred over highly dense materials suchas solid blocks of fused alumina.

It is also possible, according to this invention, to apply the coatingfrom a micropyretic slurry onto a skeletal substrate as taught in U.S.Pat. No. 4,600,481, to produce an adherent and permanent refractoryaluminium-wettable coating throughout the skeletal substrate.

The substrate may consist of blocks that can be fitted together to forma cell bottom of an aluminium production cell, or packed particulatematerial forming a cell bottom. When a carbonaceous substrate is used,it will act to carry current to the cathodic pool if there is one, or toa thin layer of aluminium through the refractory coating in drainedcells. When a refractory substrate is used, the aluminium-wettablerefractory coating assists in maintaining a shallow pool of moltenaluminium which needs to be only deep enough to permit good currentdistribution. In this case separate current conductors are providedthrough the refractory cell bottom for the supply of current, e.g. asdisclosed in U.S. Pat. No. 5,071,533 with the possible improvement thatthe tops and sides of the current feeders may also be coated withrefractory material as disclosed herein.

Steel, cast iron or other metallic alloy substrates, coated according tothe invention with a refractory coating, can be used as cathodic currentfeeders extending through a refactory bottom of an aluminium productioncell or can be coated with a refractory coating suitable for anodicapplications.

The micropyretic slurry which is the precursor of the aluminium-wettablerefractory coating may be applied in one or more layers directly to thesubstrate or onto a non-micropyretic sub-layer applied in one or morelayers on the surface of the substrate.

The non-micropyretic sub-layer may be one or more coatings of a slurryof particulates of pre-formed materials compatible with the substrateand with the aluminium-wettable refractory coating. In particular, thesub-layer may contain pre-formed aluminium-wettable refractory materialwhich is the same as that in the aluminium-wettable refractory coating,and it may also contain other refractory additives which may also bepresent in the aluminium-wettable refractory coating. Thus, thenon-micropyretic under or bottom layer(s) may be produced by applying aslurry similar to the micropyretic slurry, except that it does notcontain the micropyretic reactants.

The invention also concerns a component of an aluminium production cellwhich in use is subjected to exposure to molten electrolyte and/or tomolten aluminium or corrosive fumes or gases, the component comprising asubstrate of a carbonaceous, ceramic or metallic material, a cermet, ora compound coated with a refractory material comprising at least oneboride, silicide, nitride, carbide phosphide, aluminide or oxide of atleast one of titanium, zirconium, hafnium, vanadium, silicon, niobium,tantalum, nickel, molybdenum and iron or mixtures thereof, finely mixedwith a refractory compound of at least one rare earth, in particularceria or yttria, possibly together with other refractory oxycompoundssuch as alumina or oxides, nitrides, carbides, silicides, aluminides ofat least one of the above-listed elements or silicon, as such or incolloidal form.

The preferred refractory coatings have the following attributes:excellent wettability by molten aluminium, excellent adherence to manydifferent substrates, inertness to attack by molten aluminium andcryolite, low cost, environmentally safe, ability to absorb thermal andmechanical shocks without delamination from the anthracite-based carbonor other substrates, durability in the environment of an aluminiumproduction cell, and ease of application and processing. The coatingsfurthermore have a controlled microporosity depending on the size of theparticulate non-reactants as well as the thermal conditions during themicropyretic reaction along the combustion front.

When these refractory coatings are applied to a substrate, for instanceof graphite or anthracite-based carbon, refractory material or steelused in an aluminium production cell in contact with the moltenelectrolyte and/or with molten aluminium, the coating protects thesubstrate against the ingress of cryolite and sodium and is in turnprotected by the protective film of aluminium on the coating itself.

The invention also relates to an aluminium production cell comprising acoated component as discussed above as well as a method of producingaluminium using such cells and methods of servicing and/or operating thecells.

A method of operating the cells comprises:

producing a cell component which comprises a substrate of carbonaceousor refractory material or a metallic alloy and a protective coating ofrefractory material, by applying to the substrate a micropyreticreaction layer from a slurry containing particulate reactants preferablyin a colloidal carrier, and initiating a micropyretic reaction;

if the micropyretic reaction is initiated and its preparation completedoutside the cell, placing the coated component in the cell so thecoating of refractory material will be contacted by the cathodicallyproduced aluminium, and/or the molten electrolyte, and/or theanodically-released gases; and

operating the cell with the coating protecting the substrate from attackby the cathodically-produced aluminium, by the molten electrolyte and bythe anodically-released gases with which it is in contact.

The component may be a current-carrying component made of metal, metalalloy, or an intermetallic compound, for example a cathode, a cathodecurrent feeder, an anode or an anode current feeder. Or the componentmay be a bipolar electrode coated on its cathode face, or on its anodeface, or both.

In operation of the cell the component may be exposed to corrosive oroxidising gas released in operation or present in the cell operatingconditions, such component comprising a substrate of carbonaceousmaterial, refractory material or metal alloy that is subject to attackby the corrosive or oxidising gas and a coating of refractory materialprotecting it from corrosion or oxidation.

It is advantageous for the component to have a substrate of low-densitycarbon protected by the refractory material, for example if thecomponent is exposed to oxidising gas released in operation of the cell,or also when the substrate is part of a cell bottom. Low density carbonembraces various types of relatively inexpensive forms of carbon whichare relatively porous and very conductive, but hitherto could not beused successfully in the environment of aluminium production cells onaccount of the fact that they were subject to excessive corrosion oroxidation. Now it is possible by coating these low density carbonsaccording to the invention, to make use of them in these cells+ insteadof the more expensive high density anthracite and graphite, takingadvantage of their excellent conductivity and low cost.

The component advantageously forms part of a cathode through which theelectrolysis current flows, the refractory coating forming a cathodicsurface in contact with the cathodically-produced aluminium. Forexample, it is part of a drained cathode, the refractory coating formingthe cathodic surface on which the aluminium is deposited cathodically,and the component being arranged usually upright or at a slope for thealuminium to drain from the cathodic surface.

Operation of the cell is advantageously in a low temperature process,with the molten halide electrolyte containing dissolved alumina at atemperature below 900° C., usually at a temperature from 680° C. to 880°C. The low temperature electrolyte may be a fluoride melt, a mixedfluoride-chloride melt or a chloride melt.

This low temperature process is operated at low current densities onaccount of the low alumina solubility. This necessitates the use oflarge anodes and corresponding large cathodes, exposing large areas ofthese materials to the corrosive conditions in the cell, such largeexposed areas being well protected by the refractory coatings accordingto the invention which are just as advantageous at these lowertemperatures.

The refractory coatings find many applications on account of theirexcellent resistance, protection, and stability when exposed to thecorrosive action of liquids and fumes existing in the cell or formedduring electrolysis even when the temperature of operation is low as inthe Low Temperature electrolysis process for the production of aluminium(see for example U.S. Pat. No. 4,681,671).

MICROPYRETIC REACTIONS

The invention is based on the use of a micropyretic slurry, which whenignited starts a micropyretic reaction.

Micropyretic reactions are already known. A micropyretic reaction is asustained reaction with formation of condensed matter, starting withfinely divided particulate reactants which during the reaction are insolid state or in suspension in a liquid. The combustion takes placewithout a gaseous reactant and usually without gaseous reactionproducts. The reactants are most often in elemental form, but may becompounds, e.g. nitrides, when nitrides are desired in the reactionproducts. Micropyretic reactions are exothermic and can be initiated ina point or zone ignited by bringing the reactants to the reactiontemperature. In micropyrenic reactions, ignition starts a sustainedreaction with formation of the condensed matter, this sustained reactionproceeding along a combustion front whose propagation can be controlledby choice of the reactants, the non-reactants or fillers and thecarriers, which are the liquid portion of the slurry. Such reactions areself-propagating and are sometimes known in the literature as combustionsynthesis (CS) or self-propagating high-temperature synthesis (SHS). Twomodes of micropyretic heating reaction are recognized. One where heatingis at one point and propagation is very apparent (called theself-propagating mode), the other where propagation needs assistance(called the thermal explosion mode).

Almost all known ceramic materials can be produced by combustionsynthesis, but not necessarily without unwanted impurities. It has beenpointed out that considerable research is needed and that majordifficulties are encountered in achieving high product density andadequate control over the reaction products (see for example H. C. Yi etal in Journal Materials Science, 25, 1159-1168 (1990)).

SHS techniques using pressed powder mixtures of titanium and boron;titanium, boron and titanium boride; and titanium and boron carbide havealso been described (see J. W. McCauley et al, in Ceramic Engineeringand Science Proceedings, 3, 538-554 (1982)).

Reactions using titanium powders to produce TiC, TiB₂ or TiC+TiB₂ havealso been studied. The compact density of the reactant powder was foundto be a major factor in the rate of reaction propagation (see R. W. Riceet al, Ceramic Engineering and Science Proceedings, 7, 737-749, (1986)).

U.S. Pat. No. 4,909,842 discloses the production by SHS of dense,fine-grained composite materials comprising ceramic and metallic phases,by the application of mechanical pressure during or immediately afterthe SHS reaction. The ceramic phase of phases may be carbides or boridesof titanium, zirconium, hafnium, tantalum or niobium, silicon carbide,or boron carbide. Intermetallic phases may be aluminides of nickel,titanium or copper, titanium nickelides, titanium ferrides, or cobalttitanides. Metallic phases may include aluminium, copper, nickel, ironor cobalt. By applying pressure during firing, the final product ofceramic grains in an intermetallic and/or metallic matrix had a densityof about 95% of the theoretical density.

Known micropyretic reactions by CS or SHS are not without drawbacks andare inadequate to produce adherent refractory coatings on carbonaceous,refractory or other substrates, in particular for use as cell componentsin aluminium production, which the invention has succeeded in producing,starting from micropyretic slurries of special composition as describedherein.

The application of micropyretic reactions to produce net-shapedelectrodes for electrochemical processes, in particular for aluminiumproduction, is the subject of co-pending U.S. patent application Ser.No. 07/648,165 and Ser. No. 07/715,547, the contents of which areincorporated herein by way of reference. In said applications, a mixtureof particulate or fibrous combustion synthesis reactants withparticulate or fibrous filler materials and a particulate or fibrous,non-reactant, inorganic binder is used to produce a bulk electrode bycombustion synthesis.

The present invention provides unexpectedly good results by using anovel micropyretic slurry of particulate reactants possibly withparticulate or fibrous diluents and non-reactant filler materials whichis advantageously applied to a carbonaceous, refractory or metallicsubstrate before initiating the reaction. This slurry when ignitedstarts a self-sustaining reaction, along a combustion front, to producethe refractory material, the components of the slurry and the refractorymaterial produced forming condensed matter along the combustion front asthe reaction proceeds. The produced refractory material is usuallyselected from the group of borides, silicides, nitrides, carbides,phosphides, aluminides or oxides, and mixtures thereof, of at least onemetal selected from titanium, zirconium, hafnium, vanadium, silicon,niobium and tantalum, nickel, molybdenum, chromium and iron, as well asmetal alloys, intermetallic compounds, cermets or other compositematerials based on said metal or mixtures thereof or mixtures with atleast one of the aforesaid compounds. The refractory borides oftitanium, zirconium, hafnium, vanadium, niobium and tantalum, orcombinations thereof with the other listed materials are preferred.

THE MICROPYRETIC SLURRY

The micropyretic slurry comprises particulate micropyretic reactants incombination with optional particulate of fibrous non-reactant fillers ormoderators in a carrier of colloidal materials or other fluids such aswater or other aqueous solutions, organic carriers such as acetone,urethanes, etc., or inorganic carriers such as colloidal metal oxides.

The colloidal carrier--usually colloidal alumina, colloidal silica,colloidal yttria or colloidal monoaluminium phosphate and usually in anaqueous medium--has been found to assist in moderating the reaction andconsiderably improve the properties of the coating. It is however notnecessary for all of the applied layers of the slurry to have acolloidal carrier. Excellent results have been obtained using someslurries with a colloidal carrier and others with an organic solvent.Combinations of a colloidal carrier in aqueous medium and an organicsolvent have also worked well.

The micropyretic combustibles may comprise components to produce, uponreaction, borides, silicides, nitrides and aluminides, and mixturesthereof, of titanium, zirconium, hafnium, vanadium, silicon, niobium andtantalum, nickel, molybdenum, chromium and iron. Mostly, these reactantswill be in the elemental form, but may also be compounds, for examplefor the production of nitrides. The reactants are preferably finelydivided particulates comprising elements making up thealuminium-wettable refractory material produced. The reactants arepreferably in the stoichiometric proportions necessary to produce thedesired end products without leaving any residual reactants.

Titanium diboride will henceforth be described by way of example as thefinal material, starting from elemental particulate titanium and boronin equimolar proportions in the micropyretic reaction slurry. It willreadily be understood that other refractory compounds and mixtures canbe produced in similar manners by using the appropriate startingreactants and adjusting the parameters of the production process.

The micropyretic reaction slurry may also comprise non-reactant fillerssuch as pre-formed particulates or fibers of the desired refractorymaterial being produced, for instance, pre-formed particulate titaniumdiboride together with elemental titanium and boron. Other inert fillerswhich may be desirable to moderate the micropyretic reaction and/or toenhance the properties of the end product may also be included.

Such fillers thus are advantageously included in combination withcolloids in a liquid carrier for the reactants, such as colloidalalumina, colloidal yttria, colloidal ceria, colloidal phosphates inparticular colloidal monoaluminium phosphate, or colloidal silica. Moregenerally, colloids of other elements may be included, alone or incombination. These products do not take part in the reaction, but serveas moderators, and contribute to the desired properties of the endproduct. All of these colloids act as carriers for the particulatemicropyretic combustible slurry or for the non-reactant slurry.

The solvent of the carrier for the reactant or non-reactant slurry maybe an organic solvent in particular a urethane-based solvent such aspolyurethane, acetone but also water or aqueous solutions, possiblytogether with monoaluminium phosphate.

Other organic solvents, especially for use in combination with colloidsinclude isopropanol, ethyleneglycol, dimethylacetonide andmono-n-propylether.

The use of organic solvents which are carbonised during the micropyreticreaction can be particularly advantageous on carbonaceous substrates,e.g. due to the formation of glassy or vitreous carbon which assistsbonding of the coating to the carbonaceous substrate. Organic materialssuitable for producing glassy carbon include polyurethane/furan resins,polyacrylonitrile, cellulose pitch, vinyl alcohol, thermosetting resins,etc. Other usable polymers include polyacrylamide and other derivativesof polyacrylic acid, soluble aromatic polymers such as aromaticpolyamides, aromatic polyesters, polysulfanes, aromatic polysulfides,epoxy, phenoxy or alkyde resins containing aromatic building blocks,polyphenylene or polypheyhlene oxides. Heteroaromatic polymers such aspolyvinylpyridine, polyvinylpyrrolidone or polytetrahydrofurane can alsobe used as well as prepolymers convertible to heteroaromatic polymers,for instance polybenzoyazotes or polybenzimidazopyrrolones. Polymerscontaining adamantane, especially the above-mentioned prepolymerscontaining adamantane units, may also be used. For instance,polybenzimidazopyrrolidone (pyrrone) and adamantane basedpolybenzoxyzote (PBO) can be used in a solution of N-methyl pyrrolidone.Such polymers are pyrolised during the micropyretic reaction to formsemiconductive polymers and/or glassy forms of carbon, which adhereespecially well to carbonaceous substrates although excellent resultsmay be obtained too on other substrates such as ceramic or metallic.

Surprisingly, when using organic solvents, superior results have beenobtained when the slurry with the organic solvent is applied on top ofone or more underlayers of a slurry with a non-organic solvent, usuallyone containing a colloidal carrier.

The components of the slurry thus consist of the particulate reactants,optional particulate or fibrous fillers and the carrier which is usuallya colloidal carrier and which may, e.g. as in the case of monoaluminiumphosphate and organic carriers, be transformed or react during themicropyretic reaction.

The particulates usually have a maximum dimension not exceeding about100 micrometers, more often 50 microns or less. The fillers can beparticulates of similar dimensions, or may be fibrous in which case theymay be larger than 100 microns.

The colloids are submicronic; their particles are of the order of ananometer.

It has been found that when well-chosen slurries are ignited afterdrying, a controlled micropyretic sustained reaction takes place toproduce an intimate mixture of the resulting reaction product with thefillers and the carriers, e.g. titanium diboride or other refractorycompounds with desirable quantities of aluminium, ceria, yttria,alumina, silica or other materials including glassy carbon or otherforms of carbon which do not detract from the wettability of thematerial by molten aluminium, but usually improve the adherence and theprotection. Such materials are particularly advantageous when formed ascoatings on a carbonaceous or ceramic substrate, though, as mentioned,adherent, protective coatings can be applied also to metallicsubstrates.

PRODUCTION OF MICROPYRETIC REACTION

The production of a refractory material as a coating on carbonaceous,ceramic, metallic or other substrates involves the application of themicropyretic slurry of particulate reactants, particularly in acolloidal carrier, alone or along with particulate or fibrous fillers,directly on the substrate or onto a non-reactive sub-layer or sub-layersdevoid of particulate micropyretic reactants but which may include apre-formed particulate of the refractory material being produced and/orother particulate or fibrous non-reactants. The sub-layer(s) is/arepreferably also applied with its or their particulates suspended in acolloidal carrier in an aqueous or organic solvent.

The reactant coating may be formed by applying one or more layers of themicropyretic slurry each from about 50 to 1000 micrometers thick, eachcoating being followed by drying before applying the next layer. Thesame applies to the sub-layer which can be built up by applyingsuccessive coatings, each followed by an at least partial drying.Application in multiple layers improves the strength of the coatingafter drying and before combustion (the so-called green strength), andthis leads to better properties in the end product including uniform,controlled pore size and distribution and greater imperviousness.

These layers can be formed by any convenient technique includingpainting, dipping, spraying and slip-casting. The drying can be carriedout by air drying at ambient temperature or above, or by pre-heating thesubstrate, and possibly in an atmosphere with controlled humidity.

It is also possible to apply successive layers of the slurry containingthe particulate reactants, possibly mixed with particulate or fibrousnon-reactants, and layers of the slurry containing the particulates orfibrous non-reactants in a multilayer sandwich. Preferably, a reactantlayer will be on top, but it is also possible to top-coat with slurriescontaining pre-formed refractory material.

When all of the layers have been applied, it is important to allow thecoatings to dry for a prolonged period to provide coatings withoutcracks and with adequate green strength and to eliminate water and/orother low boiling point solvents. This full drying may take place in airfor several hours to several days, depending mainly on the temperatureand the humidity of the air and on the total thickness of the coatingswhich may range from about 100 micrometers to about 3000 micrometers andmore.

The combustion reaction can then be initiated by wave propagation or bya thermal explosion mode. In the wave propagation mode, the reaction isstarted from one part of the completed green coating and propagatesthrough the entire green surface. It may be advantageous to heat thesurface of the coating to a preheat temperature for instance frombetween 200° C. and 500° C. In the thermal explosion mode, the reactionis started by heating the entire surface of the coating and possibly thesubstrate to the required temperature to initiate the combustionreaction at all locations in the coating.

Usually the wave propagation mode is more practical. This uses a torch,laser, plasma, the passage of electric current, or any other suitablemobile heat source to initiate the micropyretic reaction and to helpsustain the reaction if necessary, or that can be moved over the coatingat a desired scanning rate to progressively initiate the reaction overthe coating as the heat source passes by. The thermal explosion mode canemploy an induction furnace or other conventional means such as anothertype of furnace or radiant heater, and this may give better control ofthe reaction leading to more homogeneous properties.

The ignition temperature is usually in the range 500°-2000° C.,depending on the reactants. Combustion may be preceded by preheating foran adequate time, about 10-60 seconds in some cases and an hour or morein others.

The micropyretic reaction may take place in air but advantageously takesplace in a reducing atmophere containing, for example, CO₂.

In the case of the wave propagation mode, combustion progresses along afront parallel to the surface of the substrate being coated. Thetemperature reaches a peak at the combustion front. Ahead of thecombustion front, the uncombusted part of the reactants is at arelatively low temperature. Behind the combustion front, the temperaturedrops gradually.

In the thermal explosion mode, the combustion reaction is started at alllocations of the coating, and progresses rapidly in depth through thecoating.

The finished material obtained utilizing slurries of well chosencomposition and methods according to the invention adheres perfectly tothe substrate, due to the controlled progression of the reaction frontduring the micropyretic reaction and the choice of the first layer(s) ofthe slurry.

Particularly for carbonaceous substrates, it is advantageous for atleast the bottom layer or the adjacent under layer(s) of the slurrycoating to have an organic carrier which, when subjected to the heattreatment during the micropyretic reaction, is pyrolized to carbonbonding the resultant coating to the substrate.

For alumina and other ceramic or metallic substrates, excellent adhesionof the coating is obtained in a similar manner, since the coatingspenetrate into pores on the surface or between particles of thesubstrate and become anchored therein.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described with reference to the application ofcoatings to components of electrolytic cells for the production ofaluminium, especially for novel designs of these cells, as illustratedin the accompanying drawings, wherein:

FIG. 1 schematically shows an aluminium production cell with a carbonbottom and lower cell wall lining coated in accordance with theinvention;

FIG. 2 schematically shows another aluminium production cell in whichcoated carbon cathode bodies according to the invention have been placedon the cell bottom in a pool of molten aluminium;

FIG. 3 schematically shows a novel aluminium production cell in whichcarbon cathode bodies having a wedge form and coated according to theinvention have been secured on the cell bottom, and cooperate withinclined anodes;

FIG. 4 schematically shows an aluminium production cell in which carboncathodes with inclined upper faces and coated according to the inventionhave been secured on the cell bottom and cooperate with inclined anodes;

FIG. 5 is a sectional longitudinal view through part of an aluminiumproduction cell having a coated carbon current collector in a coatedrefractory cell bottom; and

FIG. 6 is a schematic representation illustrating the wave propagationmode of a micropyretic reaction.

DETAILED DESCRIPTION

FIG. 1 schematically shows a Hall-Heroult aluminium production cell ofconventional design that has been modified by providing the cell bottomwith a coating of refractory aluminium wettable material in accordancewith the invention, the upper part of the cell wall with a coating 10resisting oxidation and the lower part with a coating 9 particularlyresistant to cryolite. The cell comprises a cell bottom 1 and side walls2 of carbon enclosed in a steel lining 3. The cell bottom 1 and sidewalls 2 are made of blocks of anthracite-based carbon packed togetherand bonded by a carbon-based ramming paste. Through the bottom 1 extendsteel current feeder bars 4 connected externally to a negative bus bar.To protect the cathode current feeder bars 4 from aluminium, analuminium-resistant coating 11 is applied on their surfaces inaccordance with the invention.

Several anodes 5, conventionally blocks of pre-baked carbon, aresuspended in the cell by the usual mechanisms (not shown) enabling theirheight to be adjusted. Oxygen evolving non-carbon anodes 6 may besuspended in the cell instead of the carbon anodes 5 but do not need tobe vertically adjustable because they are non-consumable. The anodes 5and 6 dip in a molten electrolyte 7, usually a cryolite-based meltcontaining dissolved alumina, and which floats above a pool or thinlayer 8 of molten aluminium on the cell bottom. In operation, thecryolite-based electrolyte 7 is usually at a temperature of about 950°C., but the invention applies also to components used in cells withelectrolytes well below 900° C., and as low as 700° C.

According to the invention, the top surface of the carbon cell bottom 1,i.e. the entire flat top surface and at least the lower parts of theinclined side walls liable to be exposed to the molten aluminium 8, iscoated with an adherent coating 9 of an aluminium-wettable refractorymaterial, preferably a titanium diboride based material containingadditives such as alumina, ceria, yttria and/or silica. This coating 9can extend to just above the maximum level of the aluminium 8, all theway up the side walls, or up to the crust 12 of solidified electrolyte,if there is one. If required, a different coating can be used to protectthe carbon from attack by the cryolite, and a yet different coating 10can be provided on the upper part of the side walls to protect thecarbon from oxidation and the fluoride fumes.

The presence of the aluminium-wettable coating 9 means that the cell canbe operated with a relatively shallow layer 8 of molten aluminium andthe anodes 5 or 6 can be held with a small and constant gap of about20-30 mm above the aluminium layer 8. This reduced anode-cathodedistance leads to a substantial reduction in the voltage drop throughelectrolyte 7, and less heat dissipation during operation. It may thusbe possible to operate the cell without the usual crust of solidifiedelectrolyte around the periphery (especially when non-consumable anodes6 are used) or at least with a much smaller crust, indicated by 12.

The aluminium-wettable coating 9 can be applied directly to a new,unused or re-built cell bottom 1, or can be applied to a used cellbottom 1 after emptying the cell of its molten contents for servicing,and machining the top surface of the cell bottom 1 to remove damaged orreacted parts and generally to renew the exposed surface.

To produce the aluminium-wettable coating 9 and the other coatings 10and 11, several layers of primary non-micropyretic slurries and/ormicropyretic slurries with appropriate reactants and preferably withfillers, as hereinbefore or as hereinafter described in detail, areapplied for instance by brushing the reactive slurries directly onto thesurface or onto one or more under coatings of a non-reactive slurry,with drying between the application of successive layers. After finalprolonged drying, and preferably after warming up the entire surface orthat part of the surface just before the ignition front, the driedmicropyretic reaction slurry is ignited, in this case, by the wavepropagation mode by an acetylene torch, or any other suitable heatsource. This starts a self-propagating ignition front at a large, heatedpart of the surface. If necessary, an additional mobile heat source maybe used to sustain the micropyretic reaction, along the mentionedpropagating ignition front.

After formation of the aluminium-wettable coating 9, to avoid a bigthermic shock to the cell bottom 1, it is preferable not to let thetemperature of the cell bottom cool down too abruptly to the operatingtemperature (usually around 950° C., but advantageously sometimes in theregion of 680°-880° C.), or much below the operating temperature.Nevertheless, cooling possibly to several hundreds of degrees centigradebelow cell operating temperature, and if necessary even below themelting point of aluminium (660° C.), is feasible without damaging thecoating. The cell can then be started with one of the usual methods byfilling with electrolyte and aluminium and raising the temperature tothe operating temperature, e.g. by the usual means of passing currentfrom the anodes 5 or 6 to the cell bottom 1 with an adequateanode-cathode distance.

The excellent and permanent wetting of the carbon cell bottom 1 by thealuminium-wettable coating 9 means that: during operation the cellbottom 1 is protected against unwanted reactions with components of theelectrolyte 7, the cell can operate with a drained cathode, theanode-cathode gap can be decreased, and no sludge or muck can come tosettle between the aluminium layer 8 and the cell bottom 1. Theoperating efficiency is thus enhanced, the energy consumption decreased,the useful lifetime of the cell bottom is extended and there isconsiderably less toxic material to be disposed of when the cell bottommust be serviced. As a result, aluminium can be produced in a cellcoated according to the invention at substantially lower cost than in anon-coated cell of the prior art.

The cell shown in FIG. 2 has a carbon cell bottom 1 and side walls 2enclosed in a steel shell 3, and cathode current feeders 4 in the cellbottom 1, as in FIG. 1. On the carbon cell bottom 1, the cell of FIG. 2is fitted with blocks 13 of pre-baked carbon whose entire externalsurfaces are coated with the aluminium-wettable coating 9. Asillustrated in the left hand part of FIG. 2, these blocks 13 may haveinternal inserts 14 of cast iron or another heavy material which acts asballast so that the blocks 13 sink in the electrolyte 7 and in thealuminium layer 8, and rest firmly on cell bottom 1. Or, as illustratedin the right hand part of FIG. 2, the blocks 13 may be secured to thecell bottom by any convenient means, such as by reaction bonding or bymechanical means.

In use, the anodes 5 or 6 are suspended with their flat lower facesfacing the corresponding upper flat surfaces of the aluminium-wettablecoating 9 on blocks 13, with a relatively small and constantanode-cathode gap of about 25-35 mm. The upper flat surface of thealuminium-wettable coating 9 acts as a drained cathode, from which afilm of cathodically produced aluminium is constantly drained into thepool 8 of molten aluminium. The level of pool 8 may fluctuate from closeto the cell bottom 1 up to adjacent the upper flat surfaces of thealuminium-wettable coating 9 of blocks 13, whereby the product aluminiummay be tapped off periodically in the usual way.

The blocks 13 may have any convenient height depending on the desiredoperating configuration, in particular so that the anodes 5 or 6 can bemaintained close to the minimum height that they would have inconventional operation, i.e. before the blocks 13 were fitted. Forinstance, the height of the blocks 13 may be from 150-300 mm.

It is also possible to suspend the blocks 13 from the anodes 5 or 6 byattachments made of non-electrically conductive materials that areresistant to the electrolyte, for example aluminium nitride or nickelsub-oxides or alumina when the cell is operated at low temperature,which attachments also serve as spacers maintaining the desired smallanode gap. In this way, the cathode blocks 13 can be removed from thecell with the anodes 5 or 6 for periodic servicing or replacement.

As a modification of the embodiment of FIG. 2, the pool 8 of moltenaluminium could contain a packed or loose bed of pieces of refractorymaterial, or pieces of carbon with internal ballast, or skeletal bodies,whose surfaces are coated with a permanent aluminium-wettable coating 9in accordance with the invention. Such pieces, which may be of randomshapes or regular shapes such as rings, form a bed which inhibits wavemotion in the molten aluminium pool 8 and thereby enables operation witha reduced anode-cathode distance, as explained in U.S. Pat. No.4,552,630.

FIG. 3 shows another anode-cathode configuration which can be fitted ina conventional aluminium production cell like that of FIG. 1, or in acell of completely new design.

In this design, carbon prisms or wedges 20 are fitted on a carbon cellbottom 1, for instance by having bottom parts 22 embedded in the cellbottom, by being bonded by a layer 23 to the cell bottom when the cellis being built or reconstructed, or by having internal ballast 24, forinstance of cast iron, which holds them on the cell bottom. These carbonwedges 20 have inclined side faces, for instance at an angle of about45° to 10° to the vertical, meeting along a rounded top edge 21. Thewedges 20 are placed side by side, spaced apart at their bottoms toallow for a shallow layer 8 of aluminium on the cell bottom 1. The cellbottom 1 can be coated with a protective aluminium-wettable coating 9according to the invention. The edges 21 are all parallel to one anotheracross or along the cell, and the tops of the prisms remain severalcentimeters below the top level of the electrolyte 7.

The inclined side faces of wedges 20, and possibly also the bottom face,are coated with a permanent aluminium-wettable coating 9 in accordancewith the invention. These coatings 9, like that of the cell bottom 1,are applied from a micropyretic slurry as before. The reaction mixturecan be ignited by wave propagation for the cell bottom 8 or by thethermal explosion mode for the wedges when these are suitablydimensioned so they can be coated before installing them into the cell.In use, these coatings 9 on the sloping surfaces of wedges 20 formdrained cathode surfaces from which cathodically produced aluminiumdrains permanently into the pool 8. Current is supplied to the wedges 20via conductor bars (not shown, but like the bars 4 of FIG. 1) in thecell bottom 1.

Over the cathode-forming wedges 20 are fitted anodes 25, each formed bya pair of plates which fit like a roof over the wedges 20, parallel tothe inclined surfaces of wedges 20 with a small anode-cathode distanceof about 15-20 mm. At their tops, the pairs of anode plates 25 arejoined together and connected to a positive current supply. The anodeplates 25 have openings 26, for example adjacent the top of theirinclined faces, for the escape of anodically-generated gas, usuallyoxygen. The anode plates 25 are made of or coated with any suitablenon-consumable or substantially non-consumable electronically-conductivematerial resistant to the electrolyte and to the anode product ofelectrolysis, which in the case of the electrolysis of alumina utilizingnon-carbon anodes, is oxygen. For example, the plates may have a metal,alloy or cermet substrate which is protected in use by a metal oxidelayer and a cerium-oxyfluoride-based protective coating produced and/ormaintained by maintaining a concentration of cerium in the electrolyte,as described in U.S. Pat. No. 4,614,569.

Alternatively, it is possible to employ consumable carbon anodes withwedge-shaped bottoms which dip between the cathode wedges 20, the anodeshaving inclined, consumable operative surfaces facing the inclinedsurfaces of two adjacent cathode-forming wedges 20, which are maintainedwith a substantially constant anode-cathode distance by lowering theanodes at a rate to compensate for their consumption.

These designs employing wedge-shaped cathodes have several advantages.As before, the permanent aluminium-wettable refractory surfaces on thecathodes protect the carbon from attack and the cell can be operatedwith a small anode-cathode distance ensuring efficient operation. Inaddition, the design permits a very high productivity per unit area ofthe cell floor, possibly 1.5 to 2.5 times as much as in a conventionalcell.

It is also possible to use pieces of carbon or refractory materials,coated in accordance with the invention with a permanentaluminium-wettable refractory surface, as other components in aluminiumproduction cells in particular components which in use are exposed tomolten aluminium, for instance weirs or baffles, side walls etc., or ascomponents in other molten salt electrolysis cells.

FIG. 4 shows a modification of the cell of the preceding Figures whereincathode blocks 13 fixed on the cell bottom 1 have inclined upper facescoated with the aluminium-wettable refractory coating 9. The left-handpart of FIG. 4 shows blocks 13 with V-shaped faces 27 inclined downtowards a central groove 28 in which the product aluminium collects.This groove 28 can be slightly inclined towards one end to facilitatethe flow of molten aluminium into pool 8. Above the V-shaped surfaces 27of blocks 13 are anodes 5 whose bottom surfaces have correspondingV-shaped surfaces, facing the surfaces 27 with a constant anode-cathodegap.

The right hand side of FIG. 4 shows cathode blocks 13 coated with thealuminium-wettable coating 9, these blocks having top surfaces 29inclined to one side, and the anodes 5 have each a corresponding slopinglower face. In these embodiments, the sloping surfaces of the anodes 5considerably improve gas release compared to conventional pre-bakedanodes with a flat bottom. The improved gas release contributes to abetter circulation of the electrolyte 7 and helps reduce the voltageacross the cell.

FIG. 5 is a schematic representation of part of an aluminium reductioncell having a non-conductive cell bottom with a special bottom-entrycurrent feeder arrangement.

The non-conductive cell bottom comprises an alumina potlining 31contained in a steel shell 33 which is connected to external buswork.Extending vertically from the bottom of shell 33 at spaced locations area number of steel posts 34 which terminate just below the top ofpotlining 31. At its top end, each post 34 is enclosed in a cap 35 ofcarbon. As shown in FIG. 1, the cap 35 consists of a cylindrical bodyhaving a central bore 36 and a closed upper end 37. The post 34 fitsloosely in the bore 36 and is secured therein by pouring in cast iron orconductive pitch by the well known rodding process, or by force fitting.Conveniently, the caps 35 are secured to the posts 34 which may then bewelded to the bottom of shell 33. To allow for thermal expansion, thetop end of post 34 has one or more slots 38. The circular top end 37 ofcap 35 lies flush with a top layer 39 of the potlining 31. This toplayer 39 may be tamped tabular alumina and is coated with a layer 40aluminium-wettable refractory material for instance including TiB₂produced according to the invention. Likewise, the top upper end 37 andthe sides of the carbon cap 35 are coated with a layer 41 ofaluminium-wettable refractory material, for instance including TiB₂produced according to the invention. Maximum advantages are obtainedwhen both the layer 40 of refractory material and the top of carbon cap35 are both coated e.g. with TiB₂. These coatings can be appliedseparately or together by applying a coating over the entire cell bottomincluding the carbon areas 37. However, the invention also forsees thepossibility that only one of the refractory or carbon surfaces may becoated. By extending the coating 41 down the sides of the carbon cap 35,maximum protection against attack by aluminium or cryolite is obtained.

Atop the aluminium-wettable layers 40 and 41 is a layer of cathodicmolten aluminium 42, which may be about 1-4 cm thick for analuminium-wettable cell bottom surface. Above the cathodic aluminium 10is a layer of electrolyte 43, typically molten cryolite containingdissolved alumina at a concentration well below saturation, into whichanodes 44 dip. In operation, the electrolyte 43 may be at a temperatureof about 900° C. or below.

The anodes 44 may be conventional prebaked carbon anodes (especially fordeep pool operation) or oxygen-evolving non-consumable anodes (forshallow or deep pool operation). Preferred non-consumable anodes have anelectrically conductive substrate coated with a protective surface layerbased on cerium oxide-fluoride. Such surface layers can be preserved byincluding a concentration of cerium in the electrolyte 43, as mentionedbeforehand and as described in U.S. Pat. No. 4,614,569.

The described embodiment corresponds to the retrofitting of an existingtype of cell with a steel shell bottom 33, used for supplying current.Of course, an alumina-filled potlining can be employed with differentcell base designs, for example having a solid aluminium base plate towhich posts 34 of a suitable high-temperature aluminium alloy arewelded. Such alloys should have a fusion point of about 1000° C. or inany event above the cell operating temperature.

Instead of being a cylindrical cap, the protective carbon member canadvantageously be a slab or bar having a flat top face which extendsacross the cell. A slot can be provided in such a bar to receive aplate-like current-collector core. Alternatively, there can be severalbores in the carbon to receive several current collector posts ofcorresponding shape. Also, especially for larger carbon current feederposts or bars, it may be possible to dispense with the inner steelcurrent supply bar.

The coating 9 of the aluminium-wettable refractory material can also beapplied to the surface of a steel current feeder which can be made toextend upwardly to contact the aluminium pool, through a protective,refractory lining. The steel current feeders can be posts whose top endsextend to openings in the cell bottom, or posts having at their top endsbars extending across the cell bottom.

The current feeders can also be made entirely of carbon cylinders orslabs embedded in carbon blocks from which cathode conductor bars extendto external negative busbars.

The coating 9 of the aluminium-wettable refractory material can also beused in other cell designs, for example where drained cathodes havevertical surfaces or are sloping at a small angle to vertical.

The invention will be further described in the following examples.

EXAMPLE 1

Several anthracite-based samples were coated with adherent TiB₂ layersas follows.

Reactant powders of elemental titanium (99.5% pure) and boron (92%pure), both -325 mesh (<42 micrometers) in equimolar proportions weremechanically blended for 15 minutes and, by adding various proportionsof a carrier, were formed into a slurry. The carrier was 0-50% by volumeof colloidal silica and 100-50% by volume of monoaluminium phosphate(Al(H₂ PO₄)₃). The powder/carrier ratio was varied from 1 g/ml to 2g/ml. The slurry was applied to the anthracite samples in severallayers, by dip coating or brushing with drying for 15-30 minutes afterthe application of each layer, and a final drying for a period of up toa day or more after application of the last layer. The applied coatingthickness was 0.5 to 1 mm. After final drying, the slurry-depositedcompositions were ignited using an oxyacetylene torch.

It was found that the optimum composition was around 25-40% of colloidalsilica and 75-60% of monoaluminium phosphate, but the silica contentcould be increased to about 50% by decreasing the coating thickness, byapplying multiple layers and by controlling the drying rate and thetemperature and the humidity of the atmosphere. For the optimumcompositions, a TiB₂ coating of good adherence was obtained on theanthracite samples. With lower amounts of colloidal silica the strengthof the combusted product decreased.

FIG. 6 is a diagram schematically illustrating the wave propagation modeof the combustion reaction, as used in Example 1, when layers 51 of amicropyretic reaction mixture of Ti and B are applied from a slurry ontoan anthracite sample 50. The upper part of FIG. 6 illustrates thetemperature T as a function of the distance D as the reaction proceedsalong reaction front 53 in the direction of arrow 54, leaving behind theTiB₂ product 52. Upon ignition, at the ignition temperature T_(ig), thetemperature rises abruptly to the combustion temperature T_(com), whichis the temperature at the reaction front 53. Behind the reaction front53, in the product TiB₂ 52, the temperature falls gradually, which isbeneficial for the homogeneity of the product. Ahead of the combustionfront, the temperature decreases exponentially with distance, asillustrated. This mode of propagation in the slurry-applied mixture hasbeen found to produce an excellent homogeneity of the reaction productand enhanced adherence to the substrate.

EXAMPLE 2

The procedure of Example 1 was repeated, with a first layer applied in 3coatings each 150-200 micrometers thick and drying for 20 minutes, usinga carrier of 50% by volume monoaluminium phosphate and 50% by volume ofcolloidal silica, with 1 gram of the titanium and boron reactant powdersper milliliter of the carrier.

A second layer was likewise applied in 3 coatings, but this time thecarrier was a commercially-available polyurethane paint thinner(PolyThin™) with the polyurethane and thinner in equal proportions byvolume. The sample was then dried in air for 12 hours and preheated to300° C. for 1 hour before combustion to remove the thinner. The samplewas combusted by torch after 15-30 seconds preheating to approximately200°-300° C.

After combustion, an adherent TiB₂ layer was produced. Similar coatings,but without the underlayer, did not adhere so well.

EXAMPLE 3

Example 2 was repeated except that the slurry for preparing the first(under) layer contained a mixture of Ti and B (70%-30% by weight) withNi and Al (85%-15% by weight). The Ni and Al powders were also -325 mesh(<42 micrometers). The weight proportions of the Ti+B to Ni+Al was 91%to 9%. The top layer contained only Ti and B, as before.

Upon ignition, the combustion rate and violent character of thecombustion decreased compared to Example 2. An adherent coating of TiB₂having a TiB₂ underlayer finely mixed with Ni and Al was obtained.

EXAMPLE 4

Example 3 was repeated except that in this case, the ratio of Ti/B tocarrier in the top layer was increased from 1 g/ml to 2 g/ml. Afterreaction, the sample was subjected to testing by immersion under moltenaluminium in cryolite at 1000° C. for 1 day. The coating was found toadhere well and, because completely aluminized, protected the anthracitesubstrate.

EXAMPLE 5

The general procedures of the preceding examples were repeated, butincluding pre-formed TiB₂ in the slurries used to form the under and toplayers.

The slurry for the first layer contained 83% by weight of Ti and B and17% by weight of pre-formed particulate TiB₂, 99.5% pure, -325 mesh (<42micrometers). The carrier was 100% monoaluminium phosphate, with 1 g ofthe reaction powder per milliliter of carrier. The slurry for the secondlayer was 75% by weight of Ti and B for 25% by weight of theaforementioned particulate TiB₂ in the PolyThin™ polyurethanepaint-thinner carrier (1 vol. polyurethane: 2 vol. thinner).

The first and second layers were respectively 750 and 250 micrometersthick. Each applied coat was dried for 15-30 minutes with a 12 hoursdrying period after the application of the third coats of the firstlayer, and a final drying of 24 hours.

The preformed TiB₂ was added to control the combustion and improve thestrength of the coating before combustion. After combustion, an adherentcoating of TiB₂ was obtained.

EXAMPLE 6

Example 5 was repeated using, as carrier for the first layer, themixture of monoaluminium phosphate and colloidal silica mentioned inExample 1, in the volume ratio 75:25. The product had a well-adheringTiB₂ coating on the anthracite sample and was subjected to testing byimmersion in cryolite at 1000° C. for 1 day. The coating was found toadhere well and protected the anthracite substrate.

EXAMPLE 7

A first layer about 200 micrometers thick was produced as above byapplying a single coat of a slurry of 90% by weight Ti and B and 10% byweight of TiB₂ in monoaluminium phosphate, with 2 g of the powders permilliliter of carrier.

A second layer was applied in two coats each about 400 micrometers thickfrom a slurry of 70% by weight of Ti and B and 30% by weight of TiB₂ inthe previously-mentioned polyurethane paint-thinner carrier withpolyurethane/thinner in equal volumes. Drying between each coating was20 minutes followed by final drying for 24 hours in air and preheatingat 300° C. for 1 hour. A well adhering coating of TiB₂ was obtained.

EXAMPLE 8

Example 7 was repeated but with two first layers each about 250micrometers thick and a single second layer about 500 micrometers thick.The ratio of the particulate of the second coating slurry was 60% byweight Ti and B, and 40% by weight of TiB₂.

A good product was obtained, although the combustion was less continuousthan with Example 7.

EXAMPLE 9

Example 7 was repeated including some silica in the first layer by usingas carrier for the slurry a 75/25 volume mixture of monoaluminiumphosphate and colloidal silica. The addition of colloidal silicadecreased the combustion rate and led to a product with good adherence.

EXAMPLE 10

Example 9 was repeated with a slurry for producing the second coatingwhich contained 60% by weight of Ti and B and 40% by weight of TiB₂. Thethickness of the second coating was reduced to 500 micrometers, appliedas a single layer.

After combustion, a well adhering coating of TiB₂ was obtained.

EXAMPLE 11

An anthracite-based cathode sample was coated with an adherent layercontaining TiB₂ as follows.

A base layer of pre-formed particulate TiB₂, 99.5% pure, was applied toan anthracite cathode sample in three coats using a solution of 25 gTiB₂ -325 mesh (<42 micrometer) in 10 ml of colloidal alumina containingabout 20% of the colloid. Each coating had a thickness of 150±50micrometer, and was dried for 10 minutes before applying the nextcoating.

A top layer of a micropyretic slurry containing particulate titanium andboron as reactants with pre-formed particulate TiB₂ as diluent and acarrier was then applied. The powder mixture was made up of 11.2 g (56%by weight) of particulate titanium, 99% pure, 4.8 g (24% by weight) ofamorphous particulate boron, 92% pure, and 4 g (20% by weight) ofpre-formed TiB₂, 99.5% pure, all these powders having a particle sizecorresponding to -325 mesh (<42 microns).

The carrier was 5 ml (14.3% by volume) of colloidal alumina and 20 ml(57.1% by volume) of colloidal yttria with 10 ml of polyurethane (28.6%by volume).

A single coating of this micropyretic slurry was applied on thepre-applied and dried base layer, providing a top layer having athickness of 150±50 micrometer.

The micropyretic slurry coated on the anthracite cathode sample was thenignited by applying a combustion torch in air. The ignition temperaturewas about 600° C. and the combustion temperature was above 1500° C.

The resulting coated anthracite cathode sample had an adherent coatingof TiB₂. Microscopic analysis of a cut specimen revealed a compact TiB₂layer adhering firmly to the anthracite substrate.

When tested as cathode in a laboratory aluminium production cell, thesample showed perfect wettability with molten aluminium (0° contactangle) and no sign of deterioration. The aluminium was found topenetrate the coating and remain there.

EXAMPLE 12

Another anthracite-based cathode sample was coated with an adherentlayer of TiB₂ as follows.

A base layer of pre-formed particulate TiB₂ was applied to theanthracite sample in two coatings using a solution of 25 g TiB₂ -325mesh (<42 micrometer) 10 ml of colloidal alumina as in Example 11. Eachcoating had a thickness of 500±50 micrometer, and was dried for 15-30minutes before applying the next coating.

A top layer of a micropyretic slurry containing particulate titanium andboron as reactants with pre-formed particulate TiB₂ as diluent and acarrier was then applied. The reactant mixture was the same as inExample 11, but the carrier in this case was 10 ml (33.3% by volume) ofcolloidal alumina, 10 ml (33.3% by volume) of colloidal yttria and 10 mlof polyurethane (33.3% by volume).

Two coatings of this micropyretic slurry each 500±100 micron thick wereapplied to the pre-applied and dried base layer, with a drying timebetween the two coatings of 15-30 minutes.

The micropyretic slurry coated on the anthracite-based sample was thenignited by applying a combustion torch in air.

The resulting coated anthracite cathode sample had an adherent coatingof TiB₂. Microscopic analysis of a cut specimen revealed a compact TiB₂layer adhering firmly to the anthracite substrate.

When tested as cathode in a laboratory aluminium production cell, thesample showed perfect wettability with molten aluminium (0° contactangle) and no sign of deterioration.

EXAMPLE 13

Another anthracite cathode sample was coated with an adherent layer ofTiB₂ as follows.

A first layer of a micropyretic slurry containing particulate titaniumand boron as reactants with pre-formed particulate TiB₂ as diluent and acarrier was applied to the anthracite sample. The powder mixture was thesame as in Example 11, but the carrier was 5 ml (25% by volume) ofcolloidal silica, and 15 ml (75% by volume) of monoaluminium phosphate.

A top layer of another micropyretic slurry containing particulatetitanium and boron as reactants with pre-formed particulate TiB₂ asdiluent and a carrier was then applied. The powder mixture was the samebut in a carrier of 10 ml of colloidal ceria.

The micropyretic slurry coated on the anthracite sample was then ignitedby applying a combustion torch in air.

The resulting coated anthracite cathode sample had an adherent coatingof TiB₂. When tested as cathode in a laboratory aluminium productioncell, the sample also showed perfect wettability with molten aluminium(0° contact angle) and promising performance.

EXAMPLE 14

A first layer about 1 mm thick was prepared from a slurry of Ti and Bpowders, as before, in a 70%:30% weight ratio, mixed with particulateTiB₂ in a ratio of 80% by weight of Ti and B to 20% by weight of TiB₂,in a carrier of 3 volumes monoaluminium phosphate for 1 volume ofcolloidal silica. 20 grams of the particulates were suspended in 20milliliters of the carrier.

A second layer also about 1 mm thick was applied from a slurry of 80% byweight of Ti and B and 20% by weight of TiB₂ in colloidal ceriumacetate. 20 grams of the particulates were suspended in 40 millilitersof the carrier.

The drying time between the layers was 30 minutes with final drying for24 hours in air before combustion.

EXAMPLE 15

Example 14 was repeated but with a 1 mm thick underlayer formed from aslurry of 25 g of TiB₂ powder in 10 ml of colloidal alumina.

The coating procedure was the same as before except the slurry forforming the first layer was held mixed for 6 hours before coating. Forboth Examples 15 and 16, excellent adhering TiB₂ coatings were obtained.

EXAMPLE 16

Example 14 was repeated but 5% of aluminium was added to the two layersby including aluminium powder -325 mesh (<42 micrometers) to therespective slurries. To prevent drying cracks, the drying time had to beincreased and the drying done very carefully. The resulting TiB₂ coatingshowed excellent wettability by molten aluminium.

EXAMPLE 17

Example 11 was repeated but 5% of aluminium was added to the two layers,as in Example 16, and the drying was carefully controlled to preventcracks. The resulting TiB₂ coating showed excellent wettability bymolten aluminium.

EXAMPLE 18

The coating of example 11 was aluminized by dipping in molten aluminiumwith different fluxes sprayed on top of the melt. The fluxes containedfluoride and/or chloride of lithium and/or sodium. The Kester 1544 fluxis available from Kester Alloys, Chicago. The Harris Brazing flux isavailable from J. W. Harris & Co, Cincinnati, Ohio. The Table belowshows the test conditions and results:

    ______________________________________                                        Flux           Temp (C.) Time    Results                                      ______________________________________                                        Kester 1544    1000      2 hrs   partially wetted                             Kester 1544 + Cryolite                                                                       1000      6 hrs   mostly wetted                                Harris Brazing Flux                                                                          1000      2 hrs   partially wetted                             Cryolite       1000      3 hrs   wetted                                       Borax           700      2 hrs   wetted                                       ______________________________________                                    

Normally, aluminization of a surface by exposure to molten aluminium maytake as long as 50 hours. Partial wetting or wetting of the surfaceafter only a few hours exposure under the flux provide an aluminizedsurface which, when it is later exposed to molten aluminium, wetsreadily. It is possible to assist aluminization by vibrating the sampleexposed to molten aluminium under the flux, or by including aluminium inthe refractory coating as in Example 16 and 17.

EXAMPLE 19

Graphite rods were coated as in Example 11 on all sides and tested foroxidation by placing them in an air furnace. The tested coatings werenon-aluminized. The rods were weighed before and after coating, andafter the given oxidation treatment. The results are as follows:

    ______________________________________                                               Weight of                                                                     coating (g)/  Temperature &                                                                              Weight after                                Weight thickness     Time of      oxidation                                   (g)    (mm)          oxidation    (g)                                         ______________________________________                                        29.55  0 (no coating)                                                                              1000 C.° <19 hrs                                                                    0                                           25.00  1.58 (light coating                                                                         1000 C.° 19 hrs                                                                     18.65                                              ˜0.5 mm)                                                         24.53  3.65 (heavy coating                                                                          960 C.° 20 hrs                                                                     25.53                                              ˜1.1 mm)                                                         ______________________________________                                    

It can be seen that the uncoated rod was fully oxidised, whereas thecoated rods had excellent oxygen resistance. A similar test with ananthracite rod with a light (0.2 mm) coating showed only a partialoxidation. The results indicate that the less-expensive graphitematerial when coated according to the invention has superior resistanceto oxidation, and will be preferred over the more expensive anthracitefor applications where the material is exposed to oxidation.

EXAMPLE 20

To all compositions in Examples 1-19, up to 10% of aluminium powder -325mesh could be added in the slurry composition so that the aluminizingprocess on the resulting coating could be made easier and at lowertemperatures.

EXAMPLE 21

All the coating compositions in the preceding examples were found to becoatable on various metals and alloys such as Cu, Fe, Ni and theiralloys.

EXAMPLE 22

The compositions of example 11 were diluted with 10% of Lithium Oxide orChromium Mono Boride in an effort to reduce the amount of sodium ionstransfered from the melt to the anthracite. The coating containing thesematerials was found to be adherent and conductive.

EXAMPLE 23

The composition of example 22 were coated on a carbonaceous substratewhich was mostly comprised of amorphous carbon. The coating was noted tobe adherent after and before aluminizing.

We claim:
 1. A method of operating an aluminium production cell whereinaluminium is produced by the electrolysis of alumina dissolved in amolten halide electrolyte at a cathode and oxygen-containing gas isreleased at an anode, which method comprises:a) producing a component ofthe cell which component comprises a substrate of carbonaceous orrefractory material or a metallic alloy and a protective coating ofrefractory material, by applying to the substrate a micropyreticreaction layer from a slurry containing particulate reactants in acolloidal carrier, and initiating a micropyretic reaction; b) placingthe coating component in the cell whereby in operation said coating ofrefractory material is in contact with at least one of the cathodicallyproduced aluminium, the molten electrolyte, and the anodically-releasedoxygen-containing gas; and c) operating the cell with said coatingprotecting the substrate from attack by the cathodically-producedaluminium, by the molten electrolyte and by the anodically-releasedoxygen-containing gas with which it is in contact.
 2. The method ofclaim 1, wherein the substrate of said component is coated outside thealuminium production cell and the coated component is inserted into thecell.
 3. The method of claim 1, wherein said component is part of a cellwhich is coated in the cell prior to operation.
 4. The method of claim3, wherein the component is part of a cell bottom formed by an exposedarea of carbonaceous material, an exposed area of refractory material,an exposed area of a metal alloy or an expanse comprising a plurality ofexposed areas selected from carbonaceous material, refractory materialand metal alloys.
 5. The method of claim 4, wherein the slurry isapplied to the cell bottom in several layers with drying of eachsuccessive layer, and the micropyretic reaction is initiated by a mobileheat source.
 6. The method of claim 1, wherein the component is acurrent-carrying component made of metal, metal alloy, or anintermetallic compound.
 7. The method of claim 6, wherein thecurrent-carrying component is a cathode or a cathode current feeder. 8.The method of claim 7, wherein the component forms part of a cathodethrough which the electrolysis current flows, said refractory coatingforming a cathodic surface in contact with the cathodically-producedaluminium.
 9. The method of claim 8, wherein the component forms part ofa drained cathode, said refractory coating forming the cathodic surfaceon which the aluminium is evolved cathodically, the component beingarranged for the evolved aluminium to drain from the cathodic surface.10. The method of claim 9, wherein the cathode surface is upright or ata slope.
 11. The method of claim 10, wherein the anodes and cathodeshave inclined facing surfaces.
 12. The method of claim 6, wherein thecurrent-carrying component is an anode or an anode current feeder. 13.The method of claim 6, wherein the current-carrying component is abipolar electrode.
 14. The method of claim 1, wherein the component inoperation of the cell is exposed to corrosive or oxidizing gas releasedin operation of the cell or present in the cell operating conditions,said component comprising a substrate of carbonaceous material,refractory material or metal alloy that is subject to attack by thecorrosive or oxidizing gas and a coating of refractory material andbeing protected from corrosion or oxidation by the refractory material.15. The method of claim 1, wherein the component has a substrate oflow-density carbon and/or amorphous carbon protected by the refractorymaterial.
 16. The method of claim 15, wherein the component in operationof the cell is exposed to oxidizing gas released in operation of thecell, the substrate of low-density carbon being protected from oxidationby the refractory material.
 17. The method of claim 1, furthercomprising exposing the refractory material coated on the substrate tomolten aluminium in the presence of a flux assisting penetration ofaluminium into the refractory material.
 18. The method of claim 17,wherein the flux comprises a fluoride, a chloride or a borate of atleast one of lithium and sodium, and mixtures thereof.
 19. The method ofclaim 1, wherein the component is exposed to contact with thecathodically-produced molten aluminium, the refractory materialcomprising an aluminium-wettable refractory boride, nitride or carbideof titanium, zirconium, hafnium, vanadium, niobium, tantalum andmixtures thereof.
 20. The method of claim 1, wherein the molten halideelectrolyte containing dissolved alumina is at a temperature below 900°C.
 21. The method of claim 20, wherein the electrolyte is at atemperature from 680° C. to 880° C.
 22. The method of claim 20, whereinthe electrolyte is a fluoride melt, a mixed fluoride-chloride melt or achloride melt.
 23. The method of claim 1, wherein the coating materialadditionally contains oxides of lithium or potassium.
 24. The method ofclaim 1, wherein the coating material additionally contains mono-boridesof chromium.